Polyamine Curatives with Enhanced Adhesion

Dytek® BHMT (Bis(hexamethylene)triamine), produced by INVISTA, offers the opportunity to formulate polyamine curatives with enhanced adhesion to difficult surfaces such as glass and aluminum, photochemical stability, reactivity rates that can be customized using a wide range of derivatives, flexibility, and impact resistance. This article explores the chemistry and the possibilities.

Epoxy and isocyanate containing systems commonly use polyamines as curatives or as chain extenders in a wide variety of CASE applications. When formulating these systems, product developers carefully optimize multiple competing parameters, one of which is the reactivity profile. The type and number of active hydrogens and amino group plays a critical role in the reactivity profile of the final applied system. In most cases, primary amines react faster than secondary amines, which are faster than primary aromatic amines. We will talk more about secondary amines that react faster than primary amines in the next CASE study article. Until then, this table provides a good summary of reactivity rates.

Reactivity toward TMXDI (m-tetramethylxylene diisocyanate)

Ref: UOP LLC Technical Publication presented at: Polyurethanes Conference 2000 Boston, MA October 8-11, 2000

Product developers working on a formulation have many commercially available curatives to choose from. At the same time, new curatives and cross linkers give product developers new ingredients to continually differentiate their products.

In response, curative suppliers typically derivatize many fast reacting primary polyamines to modify the reactivity of the active hydrogen through steric and or electronic effects. Commercially practiced chemistry to make derivatives of primary amines include a) converting the primary amine groups to secondary amines (via reductive alkylation or Michael Addition reactions) thus increasing steric hinderance, b) blocking the active hydrogen until water is introduced as observed in ketimines and aldimines, and c) creating an aspartic ester which introduces both steric and electronic effects to slow the reactivity. Many commercially available curatives begin with fast reacting primary amines and slow the reactivity using these well-known transformations.

Even if Product developers achieve the desired reactivity profile, they also consider other properties in the final product as well as in the application of the system when developing a formulation. In particular, a spray polyurea coating must be fluid enough at spray temperature to be sprayed and mixed, and it must gel and cure fast enough so that it doesn’t sag or run when applied to non-horizontal surfaces. At the same time, it must remain flowable long enough to spread, form a smooth, defect-free coating, and wet adhere to a substrate. The cure rate must be fast but not too fast. In other words, the right chemical reactivity without good adhesion may lead to defects that will usually result in premature coating failure.

Aspartic esters are commonly used in 2K spray coatings and manually applied rolled on coatings. These molecules are prepared from primary diamines and diethyl maleate. Choice of the primary diamine plays a big role in determining the reactivity rate. A more sterically hindered primary amine results in a slower reacting aspartic ester.

Triamines become a unique starting amine for this chemistry since you can create the aspartic ester secondary amines, while maintaining a useful reactive amino hydrogen as a tool to further influence other final properties. For example, aspartic esters prepared from DETA, dipropylenetriamine, and bishexamethylenetriamine (Dytek® BHMT) – with two aspartic secondary amines and an additional secondary amine functionality – exist (respective CAS #52759-68-9,1392477-70-1, 1392477-67-6), but such curatives do not claim enhanced adhesion (US 8,137,813). In fact, the reference recommends the addition of adhesion promoters.

Patent literature discloses the use of simple aminosilanes such as 3-glycidoxypropyltrimethoxysilane or 3-glycidoxypropyltriethoxysilane as adhesion-promoting additives in polyurea systems. Using this same example, the addition of an aminosilane on the remaining secondary amine suggests a novel curative with enhanced adhesion.

Dytek® BHMT becomes an interesting starting material to utilize the same chemistry mentioned above. After converting the primary amine groups to a secondary amine, ketimine, or aspartic ester, we can utilize the central secondary amine to introduce an embedded adhesion-promoting group to enhance the reactivity toward difficult-to-bond substrates like glass or aluminum.

Preparation of these novel materials is normally done in two steps: First, Dytek® BHMT is combined with a material reactive with the terminal primary amine groups so that the primary amine ends react but not the internal secondary amine group. Second, the reaction product of the first step is combined with a silane such as 3-glycidylpropyl trialkoxysilane to link the alkoxysilane to unreacted secondary amine. The products are typically mixtures and are used as prepared, without purification.

Suggested molecules based on the three types of chemistry described above:


Ketimine chemistry: Dytek® BHMT + 2 methyl isobutyl ketone (MIBK) + 3-glycidoxypropyltrimethoxysilane

Aspartic Ester chemistry: Dytek® BHMT + 2 diethylmaleate + 3-glycidoxypropyltrimethoxysilane

Michael Addition Chemistry: Dytek® BHMT + 2 Dytek® 2PN (or acrylonitrile) + 3-glycidoxypropyltrimethoxysilane


The Aminovation Lab is adding this technology to our portfolio of developmental stage molecules because the chemical structure of Dytek® BHMT suggests these potential features and benefits:

 

  • Aliphatic structure ⇒ photochemical stability
  • Alkoxysilane functionality ⇒ adhesion
  • Primary amine end groups ⇒ customized reactivity rates
  • 6-carbon linkages ⇒ flexibility and impact resistance

To discuss this chemistry in more detail or to request samples please contact us at Dytek@INVISTA.com, or post your comments directly in the comments section.

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